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What if the tower Is obstructed by clouds?
I know towers use radar to watch aircraft, obviously. But what about ground controllers? What if their view is obstructed by a low cloud level/fog?
Is there such a thing as ground-based traffic tracking? I’m not a pilot so bear with me if I sound naive.

They have ground radar for tracking movements on the ground.

Ground radar (e.g. ASDE-X) is only used ata handful of airports. The rest would use manual position tracking/reporting. But if the clouds were low enough the tower couldn't see the ground, then odds are nobody would be able to (legally) fly, so ground traffic would be minimal to nonexistent.

So essentially the airport would temporarily close operations until the fog lifted or became light enough to see through?

@StephenSprunk According to §91.175  the airlines only need 1/2 mile visibility for their 4 engine planes and 1 mile visibility for their twin-engines. An ILS usually has 1/2 mile visibility and an LPV approach can be done with visibility as low as 1/2 mile as well. Runways are at least a mile long, so there are lots of times where the tower can’t see the ground, but operations continue.

Bakersfield gets Tule fog in the winter and they have special procedures for it: ARPT OPS NA BLO 1200 RVR WITHOUT SMGCS. ARPT OPS NOT INITIATED BLO 600 RVR OR AUTH BLO 500 RVR.  AC 120-57A - Surface Movement Guidance and Control System There are airports that have Category III approaches where visibility and ceiling can be 0/0 so if that is the case, the tower isn’t seeing out the window at all.

The deadliest accident in aviation history happened when two 747s collided on the runway in dense fog.

@JScarry "Runways are a least a mile long" isn't entirely accurate, e.g. the RNAV 24 into KLOU has LPV minimums to a 4400ft runway. But the tower is relatively close to the runway and maybe that's part of the calculation.

@Pondlife Since the takeoff rules don’t apply to GA, how about we say, “Runways that serve regional jets or larger are almost always at least 1 mile long and in areas with high density altitude are considerably longer.”

@JScarry Fair enough, I'm probably being too pedantic :-)

If cloud ceiling or visibility falls below certain minima, the ATC tower will initiate "Low Visibility Procedures (LVP)". These are different from airport to airport and depend largely on the available equipment.
Some airports have surface movement radar (SMR), also known as ground radar, which will let air traffic controllers monitor aircraft on the ground on a dedicated radar screen. At airports with SMR, ground movements are virtually unaffected by LVP.
At airports without SMR, controllers will rely on their mental image of the traffic situation to control aircraft on their ground. (Note: controllers do this even if visibility if OK. You could ask a controller to close their eyes at any point during their work, and they would still know what was going on outside the windows). Obviously, the controller will need to update their mental picture as planes move around. With limited visibility and no SMR, this will mainly be done by pilot reports. Basically, the controller will ask the pilot to inform them when they pass or reach certain parts of the aerodrome, such as a certain taxiway or runway.
One additional piece of equipment that is very common to use during LVP is stopbars. Stopbars are a row of red lights that span across a taxiway and can be switched on and off by the controller. During good visibility conditions, controllers will visually scan the runway area to make sure no aircraft inadvertently enters the runway without permission. Since such visual scanning is not available during low visibility, turning on stopbars near runway holding points serve as an extra reminder to pilots that they have not received permission to proceed.
Low visibility procedures can also include special rules or procedures that come into effect if the visibility becomes really, really bad. For example, where I work, if visibility falls below 200 metres, only one aircrat is allowed to move at the entire airport at a time. So if one aircraft is moving around, all other aircraft have to wait at the gate until that one aircraft is in the air. Obviously, this requires quite a bit of planning work from ATC, but safety always comes first.


text:
Will landing gear with wheel fairings keep spinning due to airflow?
I asked this in a side note to a previous question, but I figured it warranted its own question.
If a fixed gear aircraft is fitted with wheel fairings, even if you tapped the brakes after lift-off, would the wheels start turning again? With typical wheel-fairings I guess about one third to one quarter of the tire sticks out into the oncoming air flow, and would face perhaps 120 kts wind. All aircraft tires I've seen have grooves running along the direction of travel, with nothing going sideways that would be any obvious wind catcher, so I guess they're pretty much slicks in this regard.
Or would any 'normal' amount of friction in the bearings keep them from turning?

When flying in formation, I've witnessed landing gear with and without fairings continue to spin for the duration of the flight. I think it really all depends on the specific aircraft designs.

But they continue to spin for the duration of the flight. That does not imply they spin up due to wind.

This is a very general question. So the answer is:
Sometimes they spin, sometimes they don't.
Have you ever seen a car with a bicycle on a trunk rack, and the front wheel is spinning. Sometimes they spin fast, sometimes slow, sometimes not at all. It just depends on the air flow.
Airplane wheels are the same way, and it all depends on the airflow, the tires and the bearings. The airflow over tires sticking out from wheel pants is going to be turbulent and unpredictable. Sometimes it happens, sometimes it doesn't. I've seen both.

My plane has fixed gear with wheel pants.  When the wheels are spinning right after take off I can feel the vibration (it's a really small plane).
If I tap the brakes or wait a minute or so, the vibration stops and doesn't start again.  My guess is that there's too much friction in the wheel/axle to permit the wheels to spin.
As you've noted, there's not much on the tire to catch the wind anyway.

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text:
Why more contrails over Porto than Brisbane
I have family members hell bent on the Chemtrail conspiracy and aircraft spraying stuff on us.
I’m trying to explain it’s to do with altitude, moisture and temperature.
But I need a bit of help. We’re from Brisbane, Australia and we’re visiting Porto, Portugal. Porto has much longer and larger spread contrails than we have in Brisbane.
So my argument of “You get longer and wider contrails due to more moisture in the air” is failing on me. Brisbane (at sea level) has a lot more humidity than Porto.
So why are the contrails much longer and fatter and longer lasting in a less humid area like Porto - than Brisbane?
I’m guessing maybe the humidity is felt more in Brisbane as the moisture higher up - is lower? And up there - the humidity is somehow less?0

do you have actually more contrails over Porto than Brisbane? have you kept a log? what is your observation period? have you kept track of the meteorological details of your observations? do you have high-altitude data, and not only ground surface level data? have you cross-verified with sites like fr24.com or similar to verify that is not simply a case of "there's more flights when we look up"? (Europe tends to have a bit more flights in the air than Australia)

There's no point in trying to reason with a conspiracy theorist, they won't accept evidence. There are so many factors, one would be that there are just so many more flights over Porto than Brisbane, more flights are at lower altitudes as they are shorter hops - lower means they look bigger, etc. Save your breath.

That's not what I was saying at all @sophit. Craig says some of his family are, and he's trying to clue-bat them. I'm saying to him that it's a futile effort. They either believe it in which case no amount of logic will win out, or they don't and are using it as a way to be contrary or get attention, the best way to deal with either is just ignore it.

@GdD: oops sorry, I really missed the very first sentence of the question  Anyway I agree with you...

They actually do spray more stuff in Portugal than in Australia, that's why life's expectany is shorter in Portugal...

Humidity varies strongly with altitude and prevailing winds aloft which can be opposite winds at sea level. Of course, temperature matters too because to pull a contrail you need sub-freezing temperatures.
For the best contrails you need freezing air at altitude with as much water vapor already in it as possible. Then the water vapor in the jet exhaust will form ice crystals promptly and they will persist a long time before sublimating into invisibility.
The best way to get humid air is to blow it across a warm ocean for a few thousand miles but then you need vertical mixing to get that humidity up to 30,000 feet; the best way to get dry air is to blow it the same distance across a desert and then figure out how to mix it up to 30,000 feet too.
All these factors will make it very difficult to predict where to find the best contrails!

Thanks @niels. That kinda makes sense. What I’m. It getting though, is Porto is cold and wet. We have the cold Atlantic to the west. Sea level humidity is low. Day time temps at sea level are between 6 and 15 degrees. And yet I’m seeing long and spreading trails. In Brisbane, humidity is very high and we have the warmer pacific on our east. So would that not mean high temps at 30k. It’s always sun zero up there. Between -30 and -50 I think. With the sea level humidity high - I’d expect high level humidity to be high. Is that maybe false?  There is MUCH more traffic over Porto though.

conditions at sea level are very different than at 30k feet, and are a poor indicator of what's going on "upstairs" at those altitudes the ambient temperature is dominated by the altitude effect and surface temps matter less.

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What will happen to a modern high bypass engine at 80 000 feet with cruise speed of 0.8 Mach?
While keeping the typical cruise speed of 0.8 Mach, will the engine shut down due to "air starvation" at such high altitude? Is it possible to design a turbofan engine, which is able to provide thrust, similar to typical engines at lower altitude, with this given speed of 0.8 Mach?

This is a very vague and broad question with no specific answer. Clearly though, we have air-breathing engines that function at ~80,000 ft, e.g the U-2.

@zymhan but it was not using a high-bypass engine as the question requires.

I see two questions here: "what will happen if we take a turbofan to 80,000 feet", and "is it possible to design a turbofan to function at 80,000 feet". If my understanding is wrong, you may want to edit to clarify. If you do want to ask two questions I'd suggest asking them separately, since they are fairly different.

@kevin I addressed both in my answer.

At that altitude high bypass turbofan doesn't make much sense anymore. To make then engine run at all, you really want to use all the turbine's power to drive as big of a compressor as possible and don't have any power left to drive a fan to generate meaningful amount of thrust out of such thin air.

I think the U-2 comparison is good. Not a modern high bypass design, but it gives some ideas, that to fly very high even at relatively slow speed of 0.7 Mach, with MTOM of only 18 tonnes, with very low wing loading, you will need lots of thrust. For example, the U-2 and Embraer ERJ145 have similar total thrust, but the ERJ145 is 30% heavier than the U-2.

In a conventional turbofan engine the oxygen pressure would be too low at 80,000 ft to make combustion possible. The engine would experience a flameout at much lower altitude (maybe 50,000 ft), making a climb to 80,000 ft impossible.
However, given the compression ratio of modern turbofans (1:30 to 1:50), combustion chamber pressure would be around ground pressure and with a longer combustion chamber it should still be possible to maintain combustion.

An engine shutting down, and engine providing any small amount of thrust, and an engine providing some necessary threshold of thrust(to weight or size or similar) are three different issues. A specific existing engine and a custom engineered engine are also totally separate issues.
Yes, you can make a high bypass turbofan that operates and produces some amount of thrust at fl800 while mounted on a plane traveling at mach 0.8
The other issues can't be answered based on how general the question is.


text:
How much heavier will a fuselage be with a 10% increase in diameter?
Is there some kind of a ratio or a very general rule of thumb that describes fuselage weight increase in relation to the increase of its diameter? (All other things being equal, length etc.)
Note: I took all the data from the project work "Aircraft Design Studies Based on the ATR 72". Author: Mihaela Florentina Niţă. Direct download link is here.
According to the fragments below (page 103), the estimated fuselage mass is 2323.432 kg, its diameter is 2.77 m, wetted area is 184.593 m2, dive speed 130.076 m/s.
If we increase diameter by 10%, how heavy is going to be the fuselage and why?

Hint: C = pi D

You should in addition have been supplied with a numeric value equation containing wetted surface, dive speed and fuselage diameter, all with odd exponents. This equation is derived from a statistical analysis, giving a best fit to data from existing designs. Find it, present it here, and we can give you the answer.

@PeterKämpf Yes, I have seen much more detailed project work by the same author a long ago, but then totally forgot about it. Anyway, I have rephrased some bits and included link to the project work with more data. Please let me know if that is sufficient. Thanks and regards.

Much better now!

Since the surface of a cylinder is circumference times length, it scales linearly with its diameter, and the same goes for a fuselage. With a diameter increase of 10%, the width, height and surface area should increase by a factor of 1.1 if the length of the fuselage remains unchanged.
Now we can use Torenbeeks fuselage mass formula where mass is proportional to surface area to the power of 1.2 and the denominator in the square root which also changes by a factor of 1.1. Since all other parameters remain unchanged, the square root becomes smaller (95.346%, to be precise) and the surface area factor grows by 1.121 for a total change of 1.069 or 6.9% to 2483.73 kg.
If you wonder why one factor shinks and the other grows: The term with the fineness ratio $\frac{l_H}{w_F+h_F}$ shrinks because a fatter fuselage is structurally more efficient while the surface area increases the mass because more material goes into building it.

I take issue with the number of significant digits of your results :) I don't think Torenbeek ever intended to be accurate to the gram...


text:
Bordering towered airspaces
I'm looking for guidance on radio communications when taking off from Delta airspace and flying into Charlie that starts where the Delta ends. My aircraft can only monitor one frequency at a time. Can I just tell the Delta tower I'm flying into Charlie and request a frequency change while still in Delta in order to establish comms with Charlie tower before entering Charlie airspace? If so what would be the preferred phraseology?

Can we assume that the Delta airport is the same one served by the Charlie, or is it an outlying adjacent airfield?  And what country?  (please consider tagging US and/or FAA)

Outlying adjacent airfield in the US. Delta is KTTD and Charlie is KPDX.

@mandela008 TTD can get you cleared into the Charlie if you're headed that way. And "request frequency change" is the standard phraseology to get handed off even before you leave the delta attitude. You should know the approach frequency and have it loaded already in the standby. Hope you're enjoying the wind and the hillside on the left downwind 25!

Typically, the class D tower will tell you you to when to change frequencies and it will most likely be to the class C's approach control, not their tower. You will hear something like, "Nxxxx, contact Class C approach on 123.45" and you can respond and change frequencies at that point. You still have to follow the rules for entering class C airspace.


text:
How does an AO2 station see distant lightning when fogged in?
From local aerodrome weather this morning:

SPECI KCRQ 161448Z 19007KT 1/2SM R24/3500VP6000FT FG OVC003 20/18
A2994 RMK AO2 LTG DSNT SW AND W

How does an AO2 station observe for reporting, distant lightning when the station is ensconced in low visibility fog?  The lightning and high-based cumulus actually was there at that time, but how would the reporting station "see" it?

There is a network of lightning sensors which detect lightning strikes. You can see this information, for example, on various weather-radar apps. The network feeds back to the automated weather station and allows the AWOS/ASOS to display lightning information.
From the AIM 7–1–10d:

2. Every ASOS/AWOS will contain the following basic set of sensors:
(i) Automated Lightning Detection and Reporting System (ALDARS) (excluding Alaska and Pacific Island sites).

High-level information on ALDARS can be found, for example, in this brochure from the FAA William J. Hughes Technical Center and this column from Flying Magazine.
As for how the detectors themselves actually work: some may indeed use optical sensors, but most use antennas to detect the burst of radio frequency energy caused by lightning. NOAA's National Severe Storms Laboratory and SciJinks websites contain more information on detection systems; the Wikipedia article on lightning detection would be another starting point for more research.

Related question: How far does the lightning detector work at an AWOS station, as reported in a METAR?


text:
Why does U.S. airspace revert to class E above flight level 600?
According to Wikipedia, airspace defaults to class E below 18,000 feet AMSL (that is, all airspace below 18,000 feet AMSL is class E unless otherwise specified), whereas all airspace from 18,000 feet AMSL through FL600 is class A... but, above FL600, U.S. airspace reverts to class E.
Why does U.S. airspace revert to class E above FL600, instead of simply staying class A all the way up?

@abelenky: Quoth the Federal Aviation Administration's Aeronautical Information Manual (12 October 2017 edition), page 3-2-9 (page 137\732 of the PDF): "The airspace above FL 600 is Class E airspace."

You ask why doesn't it go all the way up... but all the way up to where? It's somewhat of an arbitrary limit, but FL600 tends to generally be the top of the troposphere, so it seems like a good choice if we're picking numbers.
One reason could be that the FAA AIM (Airman’s Information
Manual) says that the top of the service volume for the high-altitude
VORs is 60,000 feet above the VOR facility, so it seemed like a good altitude to end the restriction of Class A, that all aircraft must be on an IFR flight plan.  If you're tooling around above FL600, perhaps you don't want/need to be on an IFR flight plan (think experimental planes, rocket launches, etc.)

It could also be that when they set up the airspace, there wasn't a whole lot going on above 60,000 feet, and given that survivability at that altitude without a space suit is low, so there weren't a lot of aircraft that high, plus the top of the troposphere is around that altitude, it just seemed to be a good number. Most commercial aircraft have a service ceiling well below FL600 (the Boeing 777 ceiling is FL431), and since Class A is positively controlled airspace with everyone having to be on an IFR flight plan, aircraft above FL600 are not as common, and so it didn't make sense to spend resources (at the time) to force things that high to go through some kind of waiver process, or put the effort in maintaining the services required to provide IFR separation services for aircraft that high.
However, as we look to the future, there is a great report from Embry-Riddle about Safe Operations Above FL600 that sheds some light on what goes on above FL600 and considerations that should be taken into account going forward as the airspace starts to get more crowded with drones, and planned spaceflight and supersonic transports.

"... but all the way up to where?"  The Kerman line would seem to be a sensible upper boundary.

No, it is not a sensible boundary. There is no traffic up that high to manage.

@abelenky yeah, most airplanes used for commercial service have a service ceiling well below FL600 (777 ceiling is FL431), so if you think about what's up there above FL600 it's probably stuff that the FAA doesn't need to force on an IFR flight plan. It'd be military stuff or rocket launches, mostly.

@Canuk: The link you provided to the 777X's specifications does not have the service ceiling listed; were you perhaps meaning to link to the specifications section of the main 777 page, which does?

Average top of troposphere is FL360. FL600 is not related to any interesting property of atmosphere as it is usually somewhere in the middle of stratosphere.

Airspace definitions are primarily about how Air Traffic Control handles mostly commercial and GA traffic, with occasional military traffic. They manage traffic primarily so they don't hit each other and have a mid-air collision.  I don't know of any commercial traffic that goes above 60,000 ft.  Even the Concorde's altitude record was FL600.
Only a few rare military and space-exploration flights are capable of flight above FL600, and that is on relatively rare occasions, so there are no traffic conflicts.
Even if there were traffic conflicts, ATC's typical control mechanisms (Radio voice call and response) probably wouldn't be fast enough to de-conflict airplanes travelling hypersonic speeds, as it required at extremely high altitudes.
In one of your comments, you suggested the Kármán line (around 330,000ft) would be a sensible boundary point.
Why would the FAA spend people, time, money and resources to monitor empty space?


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Why didn't the Concorde have flaps or slats?
It's well known that the Concorde had no flaps or slats.  The lack of high-lift devices considerably reduced lift at low speeds, requiring the Concorde to take off and land at higher speeds and angles of attack than if it had been equipped with flaps and slats.  As well as greatly increasing drag (and thus fuel consumption) during takeoff and landing, this greatly increased the force carried by the main landing gear and the speed at which its tyres had to spin; as a result, if and when a tyre blew out (due to, for instance, a piece of debris left by the previous airplane to use the runway), the results would be much worse than with a tyre blowout on a subsonic airliner.
Interestingly, a "Concorde B" was planned for Concorde 217 and onwards, which would have been equipped with slats (although still no flaps); this, along with slightly larger wings and more powerful engines, would have allowed the Concorde B to dispense with afterburners, considerably increasing its fuel efficiency and range.  Unfortunately, production ended with Concorde 216, the aircraft immediately preceding the first Concorde B, so no Concorde Bs were ever built.
Why didn't the Concorde, as built, have flaps or slats?

If you have not come across it yet this website has a lot of great Concorde info.

because it doesn't have a tail elevator. when you deploy flaps the plane pitches down and there's no way to counter act that.

Concorde B was proposed to have variable leading edge droop, but no slats. A delta wing with slats was used on the F-4 Phantom II, but very few other delta wings.

(Deleting my comments; see https://aviation.stackexchange.com/questions/81023/could-one-see-stars-from-concorde-in-daytime for similar comments)

The delta wing is its own high lift device.  When operated at extremely high AoA, a strong vortex will form from the leading edge of the delta wing.  This delays separation and allows the creation of more lift to very high AoA.  Flaps and slats are not needed to take advantage of this.  However, you do need a drooped nose so the pilots can see on approach.

Why no flaps?
Flaps change the pitching moment of a wing. After all, they add lift over the full chord, so the sum of the increased lift attacks at about mid-chord, which is a quarter chord aft of the regular lift. If there is no separate tail surface to compensate for the pitching moment caused by that extra lift, the aircraft will quickly pitch nose-down and crash.
Next, flaps change camber and, therefore, are destabilizing the wing. Without a stabilizing tail, a cambered delta configuration will become unstable. The only camber which is helpful for delta wings is at the leading edge and must be compensated by a little trailing-edge up deflection of the control surfaces. Positive camber near the trailing edge is destabilizing and can only be tolerated on a flying wing with artificial stability augmentation.
Why no slats?
Slats are helpful for delaying flow separation to higher angles of attack and allow a wing to create more lift. To understand their effect, it is not enough to consider what they do to the flow around the wing, but also the effect of the wing on the slat needs to be understood. A slat is like a small wing flying just ahead, and therefore in the upwash, of the main wing. The wing will induce a very high lift on the slat and in turn see its suction peak around the leading edge greatly reduced, which helps in keeping the flow downstream attached. The comparison plot below should illustrate this effect nicely:

Figure 36 from A. M. O. Smith's paper "High Lift Aerodynamics"
But a delta wing at high angle of attack does not have attached flow on its upper side. It makes use of flow separation at the leading edge which creates a powerful vortex over the upper wing. This is called vortex lift. So for take-off and landing, deploying leading edge devices would help Concorde only a little - they are most helpful in the region just before vortex lift kicks in. This would be for subsonic cruise, for which the original Concorde wing was completely unsuitable. Adding camber at the leading edge would had increased subsonic L/D a lot, so the subsonic cruise segments (like all flight above land) and flight in holding patterns would had been much more efficient. That was not considered initially, and the lower complexity of an un-slatted wing was preferred.
With Concorde B, a span increase and the addition of variable leading edge droop (no slats!) were proposed. The picture below is taken from the original web site which has been the source for the site you linked to in your question.

Concorde B aerodynamic improvements (picture source)
If you now look how the L/D would had been improved by this, it becomes obvious where the leading edge devices helped most. The L/D improvement at take-off and landing, by the way, can be mostly attributed to the span increase.

Concorde A and B aerodynamic efficiency comparison (picture source)
This information is missing from the site you linked to, but is essential for understanding why Concorde B had variable droop added: It should help to extend range beyond the Paris-New York connection and allow more efficient subsonic flight (especially the hold at 250 kts).

Related - similar issues with the delta configuration Saab Viggen.  That aircraft did have flaps, largely owing to the extreme STOL requirements, and they were mounted on the forward canards.

@ymb1: Yes, they flew with relaxed static stability. Only for a brief time, so the increased pilot workload was considered acceptable. Since the A310 all Airbus planes do the same, but for the whole flight and with stability augmentation, so the workload is kept low.

@PeterKämpf: I wonder if they got the idea from the Concorde?

@Sean: No, not at all. This is common knowledge since the Twenties - no need for Concorde to try out relaxed stability first. But Concorde had the technology first to make relaxed stability possible.

@PeterKämpf: Still, having someone else do it successfully first could have given Airbus the confidence they needed to try it out themselves, that the state of the art in technology finally had advanced enough for that to be doable.

@Sean: Yes, I agree as far as the certification bureaucracy is concerned. The engineers always knew what works, but to get it certified is a different story. The prestige and magnitude of the Concorde development helped a lot the first time, and having this precedent helped Airbus the second time.

Because with a delta wing the trailing edge has the elevators and it's too far aft to have a flap.  They depend on wing area and the ability to operate at much higher AOAs than straight wings to get the speeds down.
As for slats, they don't increase Clmax all that much, only a little bit from the increase in chord in drooping the leading edge.  The main function of slats is to function as a convergent nozzle to inject a sheet of higher velocity air along the top of the wing, to increase the stalling AOA to allow the wing to develop higher lift by operating at higher AOAs.  A normal wing with a stalling AOA of around 15 degrees will have a stalling AOA of around 25 deg with slats extended.
Delta's already operate at very high AOAs when slow because of the giant vortex generated by the steeply swept leading edge which delays the stall.  Adding a slat doesn't provide enough additional benefit.  I can't think of any delta wing a/c with slatted LEs although maybe someone knows of some.  Leading edge flaps yes, but not slats.

No, slats don't "inject high velocity air" anywhere. Best to look at them as a separate wing flying in close formation with the proper wing. Source: This paper by A. M. O. Smith, McDonnell-Douglas. See section 5.3 for a few sobering words on that issue.

Thanks for the reference.  Very interesting stuff.  I found on Page 518 where he quotes NASA material that says exactly what I said above and explains why he disputes it.  I wonder if this is an aerodynamic debate that is really settled or are there still aerodynamicists arguing over it today?

The NASA stuff is wrong. Bernoulli should be proof enough.


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Why would landing the space shuttle on water have been unsurvivable?
For an aircraft unable to reach a runway on land, ditching in water is a common and, generally, highly-survivable option; however, for the space shuttle, according to Wikipedia, "...an ocean ditching was not survivable" (although it lists no source for this claim).  What was it about the space shuttle that made it incapable of ditching into water in a survivable manner?

Since it flies like brick why wouldn’t it sink like brick ;)

"ditching in water is a common and, generally, highly-survivable option" - I think this is quite optimistic. Perhaps for small planes, but remember the Shuttle was a large aircraft, about the length and weight of a 737. The linked WP list gives just four airliners which ditched since 1970; two had very significant casualties, and one of the other two was in a stream so shallow it effectively did a belly landing on the riverbed.

There's a big difference between a "water landing" and an "ocean landing".  Inland waters (lakes, rivers) are reasonably smooth surfaces.  The ocean very much isn't.

Just wondering how common (and how much highly survivable) ditching airplane in water actually is. I know any stats are biased because ditching is done often in emergencies (without engines, etc.), but anyway, are these claims about relatively safe and common water ditching really true even for an aircraft?

That's the shortest question you have ever asked!  Where are all the footnotes?  Are you feeling OK?

Won’t space exploration be the better place for this question?

@Valay_17: Questions about space shuttle atmospheric glide, approach, and landing are explicitly on-topic here.

@Sean Sorry my bad, it’s just that the last time I had seen a question based on space shuttle landing, it was migrated to the space exploration site.

The wiki line provides no citation nor any elaboration but is largely correct. It was found that the orbiter actually had a favorable shape to ditch,

The Langley report does state that the Orbiter shape and mass [182]
  properties are good for ditching, but given the structural problems
  and deceleration loads, that is little consolation.

This was touched upon in the Challenger report, ultimately the high loads would simply have not lead to a survivable impact which had to do with both the high approach speeds and potentially insecure cargo mass (from a deceleration standpoint).

In 1974 and 1975, ditching studies were conducted at Langley Research
  Center. Although test limitations precluded definitive conclusions,
  the studies suggested that the loads at water impact would be high.
  The deceleration would most probably cause structural failure of the
  crew cabin support ties to the fuselage, which would impede crew
  egress and possibly flood the cabin. Furthermore, payloads in the
  cargo bay are not designed to withstand decelerations as high as those
  expected, 2 and would very possibly break free and travel forward to
  the crew cabin. The Langley report does state that the Orbiter shape
  and mass [182] properties are good for ditching, but given the
  structural problems and deceleration loads, that is little
  consolation.
Orbiter ditching was discussed by the Crew Safety Panel and at Orbiter
  flight techniques meetings before the first Shuttle flight. The
  consensus of these groups was that (1) ditching is more hazardous than
  suggested by the early Langley tests, and (2) ditching is probably not
  survivable.
This view was reiterated in the September 9, 1982, letter from Griffin
  to Abrahamson:
"We also suggest no further effort be expended to study bailout or
  ditching. There is considerable doubt that either case is technically
  feasible with the present Orbiter design. Even if a technical solution
  can be found, the impact of providing either capability is so severe
  in terms of cost and schedule as to make them impractical."
There is no evidence that a Shuttle crew would survive a water impact.
  Since all contingency aborts and all first stage abort capabilities
  that are being studied culminate in a water impact, an additional
  provision for crew escape before impact should also be considered.
Astronaut Paul Weitz expressed this before the Commission on April 3,
  1986:
*"My feeling is so strong that the Orbiter will not survive a ditching,
  and that includes land, water or any unprepared surface....
"I think if we put the crew in a position where they're going to be
  asked to do a contingency abort, then they need some means to get out
  of the vehicle before it contacts earth, the surface of the earth."*

NASA did publish a study about ditching analysis of airborne vehicles here (focusing on the shuttle) its science heavy but an interesting read on the subject.

Wouldn't a flooded cabin aid egress, by reducing or eliminating the pressure differential between the inside and outside sides of the hatches/windows?

@Sean not if the flooding prevents access to the exit hatch. The last thing you want to make the crew do after a crash is have to swim to an un-opened hatch and get it open, under water as the craft sank around them.

As opposed to it being completely impossible to open due to the water pressure on the outside jamming it shut?

If the exterior pressure of the shuttle is higher than the interior the system will balance it assuming it survives the impact, and the impact does not do enough damage to equalize pressure elsewhere.

The link you provided describes a system for equalising air pressures; are you saying that it would automatically flood the cabin in the event of a water landing?

@Sean I would think, in this case a system that allowed air to flow in would also allow water to flow in

Let us continue this discussion in chat.

NASA released film of ditching tests it performed on the shuttle orbiter -- it's on youtube at https://www.youtube.com/watch?v=X9kBOmHlQws

@Tristan: Those landing-speed values look pretty suspect - wasn't the orbiter's stall speed somewhere in the vicinity of 185 kts?

@Sean Video is straight from NASA Langley.  The shuttle has landed as low as 154 kts.  Per this post from the former shuttle program manager: https://waynehale.wordpress.com/2015/07/29/pilot-error-is-never-root-cause/ -- stall speed is around 150 kts.

The shuttle lands at 220 mph and would break up. This is 40% faster than airliners with twice the energy to dissipate and the shuttle structure is a great deal lighter. The orbiters were designed to go to space, ending each mission of their 100 mission lives by landing on a smooth runway. Airliners are designed for forty to a hundred thousand landings or more and they don't always fare that well in water either.

(Youtube - Ethiopian 961)

Many more than 10 landings; the shuttles averaged about 35 landings each before they were retired, and could have managed more.

@RussellBorogove Updated, thanks, but the point still holds. I found a JPL reference.

Re "don't always fare that well in water", that's far more a matter of the crewt trying to control the plane while fighting off hijackers, than of ditching.  The same "landing" on an actual runway would have been as bad, if not worse (as the wreck might have caught fire).

The didn't call it a "Flying Brick" for nothing. There isn't much margin of control for landing safely on a normal runway (the actual shuttle landing strip is longer than "normal"), let alone a water landing.

@jamesqf They weren't actively fighting off hijackers at that point. The hijackers sort of gave up at that point when they realized that the plane had actually run out of fuel. At that point it wouldn't have mattered if there were hijackers in the plane or not.

@forest: The Wikipedia article atates otherwise: https://en.wikipedia.org/wiki/Ethiopian_Airlines_Flight_961  And if the pilot was not fighting hijackers (or otherwise distracted), then why does the plane bank at the last second, causing one wing to strike the water first?  My point is that regardless of what actually caused the wing strike, the plane would also have crashed if it had happened on land.

@jamesqf That article says that a fight at the end caused the pilot to lose sight of the shoreline. That wasn't what caused it to bank suddenly to the left (that was the fault of a plane with no engine power trying to land on the ocean). But I agree with your main point.

@forest: Why would having no engine power cause a bank, though?

@jamesqf Planes tend to get rather hard to fly when you're very low, very slow, under a lot of stress, and have no engines. If you read the accident report, it explains that a fight with the hijackers only caused the pilot to lose sight of the shoreline when they were still fairly high up in the air (which is what forced the water landing).

@forest: Losing one's visual reference does tend to cause the attitude of one's aircraft to go pear-shaped.

I would still argue that Ethiopian 961 is not a good example to put here. It's not that close to a good example of a ditching.

Lowering ground speed increases survivability by the square of velocity.  In other words, an off runway landing at 100 knots has only 25% of the energy to dissipate compared with a 200 knot impact.
Deltas have the ability to maintain lift at a very high angle of attack, as demonstrated by the XF-92A holding off the runway at 67 mph, 100 mph slower than previous landings.
The inability of the Shuttle to ditch may have had something to do with its ability to re-entry the Earth's atmosphere.  The first Shuttle flight revealed a strong pitch up tendency due to the ionization of air under the nose from the extreme heat.  A forward CG adjustment helped control this.
Also the normal landing technique, for the runway, was to come in at very high speed, with a shortened nose gear, allowing the nose to drop to negative angle of attack and "stick" it to the runway after touch down.  (No bounce and go around here).
This craft simply was not designed for slow, high AOA landings on water or anywhere else. Even if it could pitch up, it's great weight would put tremendous stress on any structures while decelerating during a "ditch".
An ejectable crew capsule may have given them better odds.


text:
What prevents DC-9 series aircraft from being reengined with more-efficient engines?
All DC-9s except for the MD-90\B717 series use old, inefficient Pratt & Whitney JT8D series low-bypass turbofans, even the best of which have a paltry 1.74:1 bypass ratio.  We know that a DC-9 can accommodate a high-bypass turbofan; the final iteration of the DC-9 (the MD-90 series, including the Boeing 717) uses much more efficient engines with bypass ratios in the 4-5:1 range (which is the main thing distinguishing the 90-series aircraft from their predecessors).
Given that we know that a DC-9 is perfectly capable of using a high-bypass engine, and that the kerosene-guzzling nature of the JT8D is the main reason why DC-9s are being steadily phased out of service (as their low fuel efficiency makes it hard for airlines to make a profit from a pre-90 DC-9), why haven't the remaining older-series DC-9s been re-engined with high-bypass turbofans?

A re-engine program can be pretty expensive. This can make sense on military aircraft that take much longer to accumulate hours and cycles, or even cargo aircraft like the DC-8 in a similar position.
But the newest aircraft in the MD-80 series are going on 20 years at this point. They just don't have enough life left to justify that kind of expense. Most of them are not equipped with glass cockpits. When you add on the avionics upgrades needed, it becomes more cost effective to just buy a 737 or A320 which comes with the latest engines, avionics, and other features.
You're going to need a major manufacturer willing to take on the effort of an engine replacement program. That would point to Boeing or Airbus, and that would only cut into their existing narrow body sales.

Why would a reengine require major avionics upgrades?

@Sean I think engine control systems are different for modern engines, so the autothrottle and flight management computers would need to be upgraded to properly talk to the new engines.

It would have to be Boeing, given that they're the ones holding the DC-9 type certificates ever since they acquired McDonnell Douglas.


text:
Don’t seats that recline flat defeat the purpose of having seatbelts?
Virtually all first-class airline seats, and many to most business-class seats, recline all the way down, either as lie-flat seats (where the seat reclines to a 180º seatback-seatpan angle, but the fully-reclined seat is still tilted somewhat forwards relative to the cabin floor) or flat-bed seats (where the seat reclines to a truly flat surface that can also, as the name indicates, serve as a bed).
These seats are, of course, equipped with seatbelts, but I’m having great trouble seeing how even a tightly-fastened seatbelt would provide any protection to a flat-lying occupant, as any significant longitudinal force would cause the occupant of the seat to slide under the belt and out of the seat.1 2
Although these seats, like all airline passenger seats, are required to be locked in the fully-upright position except in cruise flight, this still leaves the occupants (especially those in flat-bed seats) vulnerable during the (sometimes quite long) portion of the flight when they are allowed to recline their seats to flatness; a moderate head-on (or tail-on, for that matter) gust would be enough to separate these occupants from their fully-reclined seats, as would any other event capable of suddenly increasing or decreasing an aircraft’s inertial speed too quickly for the “fasten seat belts” sign to provide any warning (not, as noted above, that fastening their seatbelts would actually provide any significant degree of protection to the occupants in question, but it could conceivably induce them to derecline their seats in preparation), such as a flight-control malfunction or evasive action to avoid a MAC (both of which produce large speed excursions secondary to rapid changes in attitude and altitude).
For that matter, even vertical accelerations with no longitudinal component would still be expected to injure flat-lying passengers more severely than those in an ordinary sitting position, for reasons illustrated below:

Am I missing something?

1: This is known as submarining, and is a major problem with car seatbelts (even with seats that don’t recline at all), which are much harder to fasten tightly around the pelvis than aircraft seatbelts.
2: If the belt were fastened tightly enough to pin the occupant to the seat even under considerable longitudinal force, this would

be highly uncomfortable for the occupant, and
upon the application of significant longitudinal forces, drag across the occupant’s abdomen until catching on their ribcage, causing severe internal abdominal injuries plus likely breaking the bottom few ribs.

I like your drawing.

Usually when the seatbelt sign goes on, especially if they ask the flight attendants to sit down, they'll ask people to bring the seat backs up. I've never heard of an instance of a "sudden" head wind or tail wind powerful enough to unseat passengers. Turbulence is a bigger issue and they have ways to detect that before I usually gets too bad.

Fully reclining seats are usually angled at least 30 degrees from the centerline, so if there's a sudden forward or backward acceleration passengers will go into the side of the chair even when reclined. Buy really you're never going to get enough forward or backward acceleration to matter.

"Note increased flailing." LOL

Upvoted for "Note increased flailing".

@RonBeyer that must be airline specific, as I routinely fly Dubai-Auckland in Business Class on Emirates A380, and I have never been asked to return my lie flat seat to the upright position when the crew have been asked to take their seats during cruise.

I'm not sure that comparing car seatbelts to airplane seatbelts is that useful here. The acceleration involved in a car crash at highway speeds are much greater than even the acceleration, mostly vertical, involved in even severe turbulence.
For example, after a 2015 severe turbulence encounter, investigators determined: "In the first event, the peak vertical acceleration forces recorded were + 1.7 and + 0.14 g. In the second event, peak vertical acceleration forces recorded were + 2.21 g and −1.32 g," not that much greater than the kind of acceleration you might feel in a car if you drive really aggressively. That's enough to throw unrestrained passengers and objects about (just as if you slam on the brakes in a car), resulting in injury, but it's a much smaller acceleration than what you'd see in a 30mph car crash. Nobody would consider being slammed against their seat belt like that, whether they're seated upright or reclined, to be enjoyable, but it beats slamming your head into the ceiling.
Forces much larger than that exceed the design load limits required for the aircraft, and once you're beyond that and the required safety factor, your biggest worry is the structural integrity of the aircraft, and you're less concerned about whether people's limbs flailed about.
In the case of an actual crash (such as the 16g standard new airplane seats are tested to), lying flat could pose a much larger risk of injury, but that's beyond the design specifications of the seat, which are tested with test dummies in a normal upright position. There aren't a lot of survivable crashes that occur without warning during the cruise phase of flight (page 22).
Car seats can also be reclined, and the diagrams the NHSTA produced of the results of crashes in those cases (see, for instance, page 41) are remarkably similar to yours. This is why "bring your seat to the upright position for landing" is a standard requirement.

Actually there are many accidents that occur without warning during the cruise phase of flight, because any occurrence where somebody is injured qualifies as accident and turbulence-related injuries are quite common. But there is not much on the aircraft that the turbulence could grab to shake it in longitudinal direction, so only vertical accelerations are a concern. The linked document only counts hull-losses, which is much smaller category. In cruise a non-fatal hull-loss can only happen as emergency landing and there is obviously lot of time to put the seats upright in such case.

@JanHudec Good point. I changed it to "crashes," since the point is there are few situations where you're asleep during cruise flight and are suddenly in a crash where it would make a difference whether you're sitting upright.

You would have trouble pulling two gees in a car, as tire adhesion is less than 1 gee. Seat belts are to prevent ejection from the vehicle in crashes.

@Harper Formula 1 cars can pull up to 6 g in braking, and similar laterally in high speed turns. But you're right, even the supercars available to the general public (at least its richest members) struggle to pull much more than 1 g

+1 The NHSTA study with photos is most informative. (Sadly, no mention of increased flailing:)

@Harper: Nowadays, they're mainly to keep you from getting thrown into the various hard parts of the car's interior.

Yeah, even in the strongest wind gust longitudinal acceleration or deceleration is not a problem; we are talking only fraction of a G. During cruise flight sudden turbulence causes most damage and there the vertical component is critical. Even then the experienced vertical accelerations are negligible when compared to car accidents. They are enough to toss people and equipment around if not properly secured but again, the acceleration is usually less than a G.
Airframes are certified to -1/+2,5G vertical acceleration with additional margin of 50% so structural failure is likely to occur way before car wrecking forces are encountered.

Keeping you in your seat is not only for your benefit. If the plane experiences sudden vertical motion (turbulence / air pockets etc) then people bouncing all around the cabin would be a touch messier than people "flailing" (lol) in their own seats.
I've seen this, during air pockets over Seoul. Most passengers were buckled in, but one or two passengers were up. As the plane lost altitude suddenly, the people not strapped in were flung against the ceiling, and then slammed back down to the floor as the plane's fall was stopped. Fortunately none were seriously injured, just bumps and bruises. If there had been even a slight sideways component to the plane's sudden motion then these people would not have landed in the aisle or back in their seats. They would have landed on someone or something else. Imagine a large person landing on a child or elderly person. Or anyone landing head first on an armrest.
It's not perfect, but "increased flailing" is a smidge better in a general sense than the alternative...

It's the answer I would've given +1.  Also in cars I knew a guy who swore by driving like this without a seatbelt.  You just hit the ceiling instead of flying out the front windshield.  But that was also back when a Cadillac was a Cadillac.

I reckon turbulence (i.e. vertical acceleration, pitch and roll) is the issue based on experience in the cheap seats and reports of in-flight injuries.  It's not 100% predictable, and if you're fully reclined or asleep (as you've noted) it takes time to react so some protection is probably a good idea. Loosely fastening the seatbelt keeps the passenger from being thrown off the bed and landing somewhere hard, or even just rolling over the edge of some seats (many of the first-class ones appear to be a bathtub shape with padded sides so that would be less of an issue, but others don't have much between them and the aisle).
A similar approach is used at sea: on the overnight ferries between the UK and France there are bed-belts. These don't have to double as seat-belts for take-off/landing so tend to be elasticated and often (but not always) have two fastening points on one side but only one on the other.  They're a big help if the ship is pitching and/or rolling, which of course means vertical acceleration if you're far from the axis.


text:
Why do airports in the UK have so few runways?
Almost all of the major airports in the United Kingdom are single-runway (or functionally-single-runway) installations, with only two of the very busiest (Heathrow and Manchester) having as many as two runways (although Heathrow has a third under construction).
(In addition, even for the three major UK airports that have two runways, the two runways are always a parallel set, with no provision made for crosswind operations.)
This is - with few exceptions - far fewer runways than would be normal for airports of their size.  Taking the seven airports in the UK with more than ten megaemplanements in 2018 and comparing them to similarly-sized airports elsewhere:

Heathrow (two runways, with a third under construction) v. O'Hare (seven runways, with an eighth under construction)
Gatwick (two [functionally one] runways) v. Newark (three runways)
Manchester (two runways) v. la Guardia (okay, two runways)
Stansted (one runway) v. Baltimore-Washington (two air carrier runways1)
Luton (one runway) v. Calgary (four runways)
Edinburgh (one runway) v. Norman Mineta (two runways)
Birmingham (one runway) v. Raleigh-Durham (two air carrier runways1)

Having so few runways not only increases congestion and severely limits the number of flights that can use the airport without infringing on safe separation distances between aircraft, but also poses the risk, for the four single-runway airports,2 of an unserviceable runway (for instance, due to snowplows, potholes, trespassers, flocks of birds, wayward deer, or a disabled aircraft) shutting down the entire airport, potentially for a prolonged period of time.
Why do the UK's major airports have so few runways compared to the norms for airports of their sizes?

1: Plus one general-aviation runway each for Baltimore-Washington and Raleigh-Durham.
2: And, to a lesser degree, for functionally-single-runway Gatwick, as its emergency backup runway is close enough to the main runway for a single debris event to potentially affect both.

For one thing, the USA is roughly 35 times larger than the UK. Texas alone is more than twice the size of the UK. When it comes to land use, the "norm" in the US isn't necessarily comparable to other countries, especially much smaller ones.

@Pondlife: I am aware that, due to the US's far greater size, a far greater proportion of people choose to fly (rather than drive or take a train) in the US versus in the UK.  That's why I compensated for that by specifically matching the busiest UK airports to other airports with very similar traffic levels.

I think Pondlife is referring to land use rather than average travel distance. Land value in the UK is more than double that in the US.

@Sanchises: Oh.  Good point.

Heathrow has a third under construction - not yet it doesn't. And if certain politicians get their way, there never will be. Should it go ahead, you'll note that it is (you guessed it) another east/west runway.

Possibly worth noting that in WWII a number of airbases were constructed in the UK with a 3-runway configuration, see e.g. en.wikipedia.org/wiki/Class_A_airfield

@Bob, and many GA fields still have non-parallel runways, but airliner take-off and landing speeds are probably double that of WWII and GA aircraft, so they are less affected by side-winds.

@Sean are all those runways in use at the same time though? Or they they pick the one or two that are most usable in current weather conditions?

@stripybadger: Well, for the first comparison on the list, Heathrow has two parallel runways (soon to be three), whereas O'Hare has five parallels (soon to be six).

Potentially historical - smaller runways were harder to target with bombs in WW2 and its simply stayed that way ?

If you divide the area of the UK by the total number of runways that serve international flights you will find that it actually has proportionally more runways than the US. That is: the average service area of UK runways is smaller than for US runways.

An interesting look at the history of London's airports can be found here - https://www.youtube.com/watch?v=AbAal7jIWQ4. Not really a direct answer to your question, but an interesting insight nontheless.

@Pondlife While that's absolutely true, even Schiphol (AMS) has 5 runways (6 if you include the non-international one). That's in the Netherlands in an area where land is just as much at a premium as around the large UK cities.

@Criggie not really, during the Blitz most RAF fighter command airfields were just that, fields. There were no marked runways, aircraft would take off in whatever direction was most convenient from a flat area of grass.

When Heathrow was opened as a commercial site it had THREE different runways in a triangle and by 1955 it had SIX runways - You can see them here: Wikipedia commons - arranged to allow parallel operation on any 2 runways no matter what the wind direction was.
But with the coming of larger transport aircraft having higher landing speeds and greater crosswind tolerances, the need for the extra runways was diminished - and by the end of the 1950s only the east/west runways were being used - they got extended into the 2 runways in use today, whilst the other runways were closed - they're used as taxiways today.
Heathrow is adding a third runway - east/west for capacity.
Most other airports are single runway or "functionally single runway" - which means that only one runway is ever in use at a time because that's all they need to be, or because that's what they've been limited to by planning restrictions.
Gatwick is in the middle of trying to open up its second runway for parallel operations - this was part of the original plan when it was opened, but is being fought hard by locals on noise grounds.
In other areas one of the ways to quell opposition has been for airports to quietly buy up land under the approach paths for 15-20 miles in each direction and ensure it stays as farmland.
This is not economically possible in the UK - and even if a brand new airport was to be opened away from a major city and rail/road links magicked into place for it (which have proven very hard to get established - even a decent north/south rail link into Heathrow is near-impossible), the odds of being able to acquire affected land at any price are slim to negligable.

+1 It's very difficult to get planning consent for new runways, it takes years to go through the process, the appeals, public enquiries, judicial reviews etc.  Also, Doncaster airport (the former RAF Finningley) is a great example of the point you make in the last paragraph - big airport with little traffic

Buying land even THREE miles west of Heathrow is going to be tricky.  Some of the land there last changed hands just short of a thousand years ago - and William the Bastard's current successor is not going to sell Windsor Castle now.

There are perhaps 101 non aviation-related reasons why the UK does not have larger airports, such as space consideration (we're only a small island!), politics (NIMBY!), civil engineering (Airports are commonly near urban centres and are often surrounded).
However, the most aviation-related related reason I can think is that we just simply rarely need cross runways. If you look at most large airports in the UK their runway(s) are east-west(ish). The reason for this is over 70% of the prevailing wind direction in the UK is West or SouthWest.
In addition, we rarely have extremes of wind - those exceeding the crosswind capabilities of common commercial airliners. Heavy snow is even rarer. I can probably count the number of times a runway at a large airport has been closed due to any of the reasons you mention in the last 20 years on two hands.
Bottom line; it's just not necessary. There is no socio/political/weather climate to warrant it.

Amsterdam shares has much the same climate as London has, yet Schiphol Airport has 6 runways.

@Koyovis I assume Amsterdam has much more variance in the wind direction. They usually use the 3 North-South runways (or 2 of them and 06/24), so winds are probably not "West or SouthWest" most of the time.

@Koyovis, look at the graph for dominant wind direction for AMS: https://www.windfinder.com/windstatistics/amsterdam-schiphol there is a lot of variation in wind direction and some runways are used to allow for flight paths that cause less noise polition for the surrounding citizens

@Brilsmurfffje Indeed, my point as well. I wonder how different the wind direction scatter at Heathrow is.

@Koyovis, LHR is a lot more consistent as can be seen from the graph:
https://www.windfinder.com/windstatistics/london-heathrow this is also logical given the close proximity of AMS to the ocean

OTOH old UK airports used the triangle style runways (also Heatrhow, you may still see on areal photo the old runways), so wind was a factor. -- Amsterdam has only one large airport, London has 4 (to 6). Italy seems to use the same method: an additional airport, instead of increasing the size of the larger one. I think local authorities has much more weight (let's get new job, but also: we have enough departures).

The North Atlantic Oscillation ensures that westerly winds predominate over most of Great Britain. A non-aviation consequence of this is that in most British cities, the West End is the posh district. This came from the days of coal fires, when the windward west-side had the cleaner air. Eastenders had to choke in the smog.

@Koyovis The climate is "much the same", but the devil is in the detail. For example Schiphol has an average of 26 snowy days per year. LHR has an average of 4.

@OscarBravo oh, that explains the name of that TV show with the awful theme song that always made me dive for the remote after the news was over

@OscarBravo Thank you! I've been wondering about that ever since I lived in Sheffield, years ago, and noticed the pattern.

@Koyovis Also, that might tie in with the land value point. When the Netherlands is short on land they just make more.

Possibly airports in places like the US have more GA / small airline traffic. Major UK airports discourage GA (and small airlines - it's fairly impossible to get a scheduled flight between most UK cities), Airliners are happy with a single/paired runway, so that's what gets built, with taxiways and terminals where cross runways would go.

@Brilsmurfffje: Going by that wind rose, it seems like Heathrow should have its runways oriented northeast-southwest, rather than east-west.

@Sean - I think it did, originally.   https://en.wikipedia.org/wiki/History_of_Heathrow_Airport#cite_note-18

@OscarBravo Huh, I never realised that about the West Ends. Nice!

@owjburnham: The same goes for pretty much any coastal airport (look at Logan, Kansai, Hong Kong...).

@Sean And, of course, the proposed Thames Estuary Airport!

@Rich While the U.S. does indeed have more GA traffic, most major U.S. airports tend to discourage it as well. Major U.S. airports do have far more aircraft movements than almost anywhere else in the world (last I looked pre-Covid, 8 of the top 10 by movements were in the U.S.,) but they're still mostly just passenger jets at the major airports. Just a whole lot of passenger jets. ATL and ORD both had around double the aircraft movements of LHR before Covid, for example, virtually all of which was scheduled airline flights. ATL and ORD each have 5 parallel runways.

It's ALL about land use.
Look at Mirabel, Denver's new airport, or Dulles (at the time it was built).  They basically razed some farmland.  America had the opportunity to do a whole lot of that.
Whereas in the UK, you have an ancient civilization dotted with villages, the entire system of landed [owns land] gentry (as seen on Downton Abbey; Highclere Castle has been in the same family for 300 years), and the social systems involved the land itself: the land was owned by the lords (literally "land lords"), and leased to the occupants; that's where for instance Mr. Darcy's "income exceeding ten thousand per year" came from.
And the House of Lords is still a political force.
In the UK, airport sites aren't selected, so much as gouged out of history.
So it is not lightly that they expand an airport.
Personally, I think in this kind of situation, the entire exercise is folly; they should site airports on land that could not possibly have been developed except with modern methods, such as the proposal to put London's airport in the Thames estuary.  Oh wait, there's history there too.

I've downvoted this because it uses fictional sources as references and ignores the fact that many, many airfields have been built in the UK (especially England) for military use and have since become civilian airports (and brownfield sites)

@DaveGremlin It's rather harsh to downvote for that.  The reason for fictional sources is this is what the reading public can relate to.  Why shouldn't we use readily available social metaphor to describe how things worked?  Answers should be accessible.  If you want to downvote me for that, you should be prepared to show how the concepts can be gotten across non-fictionally.  Please write your own answer and we will let the readers decide.    As far as the former military use, military exigencies can get a military airport established, but that does not help build or expand a civil one.

Also you do realize Highclere Castle is real?  So is the world of nobles and gentry portrayed in the Jane Austen books. She didn't invent the English class system.

I'm not sure why the wreck of the Richard Montgomery is brought up specifically in opposition to the idea of a Thames Estuary airport; it's going to go boom sooner or later, airport or no, and better to have it happen now and get it over with than wait until the surrounding area is even more built-up and crammed with people and development than it is now.

Why @Sean, I do believe you have the plot of a James Bond movie there! Evil genius aims to blow up the Montgomery so we can get on with building the airport, which he profits from somehow...

@Harper  Are you saying that I can only criticise points in your answer if I provide one of my own?  Whenever I read a bad novel do I have to rewrite it to be allowed to criticise it?

+1 for "gouged out of history.". An excellent description of pretty much any non-trivial civil engineering project anywhere in the UK.

Heathrow: 4.74 sq mi. O'Hare: 11.9 sq mi. - you're going to need a bigger plot. Also, there's no comparison to be made to the city that once had both the largest and busiest airports at the same time.

Sorry but this is basically a bunch of stereotypes. The House of Lords is now almost completely non-hereditary. It's hard to see how Jane Austen is relevant, given that she died most of a century before the first powered flight. And, really, it doesn't much matter who owns the land or how long they've owned it. Demolishing a section of London to expand Heathrow really isn't much different from demolishing a section of Queens to expand JFK.

@DavidRicherby: Except that you don't need to demolish any sections of Queens to expand JFK, since it's right next to Jamaica Bay and you can just make new land to expand the airport onto.

The wind in the UK is fairly constant east-west, thus there is no need to construct runways in other directions. The only reason to construct extra runways, is for the case where the capacity of the current runway system is insufficient. LHR shows that you can handle a large number of yearly flights with just two runways. LHR is currently operating at max capacity, thus there are plans for a third runway. The other airports you are mentioning are not even seeing half the traffic of LHR and are thus fine with using just one runway.
If you are currently running an operation with just one runway, your capacity can more than double if you construct another runway, since you can then use one for take-off and one for landing.
LHR has some graphs indicating how the wind influences the operation:
https://www.heathrow.com/noise/heathrow-operations/wind-direction
Airports closer to the ocean, such as Aberdeen, have more runways since the wind direction is less constant.
TLDR; dominant wind direction dictates the orientation of the runways and the number of yearly flights dictate the number of runways. This is all within political/environmental/spatial limits.

LHR does use easterly operations sometimes. It's 20 years since I worked there, but the switch between westerly and easterly operations uses up pretty much all of the slack in their scheduling. This means the fact that the wind doesn't change direction often is crucial to operations

"The wind in the UK is fairly constant east-west"  needs to be corrected.  The wind in the UK is usually a south westerly, not east to west.  However it can be from any direction.  I'm not sure what you mean by 'constant' in this context, neither the wind speed nor the direction have much constancy about them.

@DaveGremlin "East-west" here refers to both east to west and west to east (as in 09/27) as opposed to, say, 02/20.

Adding a third parallel runway usually only gives a marginal increase in capacity, because you need traffic patterns for arrival, departure and missed approach for each, which is why middle runways are rather seldom in the world.

Aberdeen has the one runway and three helipads - it's basically a huge heliport (for the oil industry) with some fixed wing facilities,

@PerlDuck - Even using that interpretation, 'east-west' is still incorrect.


text:
What are these strakes on the belly of the F-16 for?
What are these 2 things doing under the F-16? Are they for aerodynamics?

Source.

Does this answer your question?

Those are called Ventral Fins.  They provide more vertical tail area (for yaw stability, etc) particularly when the aircraft is at high angle of attack.


text:
What aircraft used this compass?
I was wondering if anyone had any info on this compass I have.

It's a magnetic compass ordered by the USAF. I don't think there is any way to figure out exactly what aircraft this specific compass was installed in. It is way too generic of a part to determine that.

Corresponding specifications, confirming a general use (§6.1).

This model compass was used on many USAF aircraft, B2, B52, F4, F18,C17, C130, and SH60 and HH60 helicopters.
Judging by the relatively low SN, I would guess B52, C130, or F4.


text:
Why do fighter aircraft not use anti-shock bodies for fuel storage?
My question bears specific reference to the anti shock bodies (also known as Küchemann carrots) used on the wings of the Convair 990 jetliner. Whilst they increased the maximum speed of the aeroplane slightly, this is nowadays achieved with supercritical wings. However, on the CV990, they could be used for additional fuel storage without the penalty of speed reduction.
So my question is why do modern military aircraft not use anti-shock bodies instead of conformal tanks (e.g. F15 and F16)?

Do the conformal tanks reduce the top speeds of aircraft such as the F-15 & F-16? If not, then it doesn't really matter, and it's likely that the conformal tanks can be made much larger. Additionally, a quick peruse of the pics of an F-16 on the Wikipedia entry don't show anti-shock bodies, so they couldn't be used for fuel storage.

Who says they didn't? You need to look further into history, though.
The first application of tanks shaped as Küchemann bodies were "Doppelreiter" fuel tanks (slipper fuel tanks) which were used on some German fighter airplanes in WW II. They were mounted above and behind the wing, and to everyone's surprise they had little impact on the top speed of the airplanes, and in case of the Me-309 helped to increase it slightly. At that time, transsonic flow was little understood and the tanks' negligible impact on drag was an unexpected benefit.

FW-190 A with Doppelreiter tanks.
More modern, supersonic fighter airplanes use a coke-bottle like constriction in their fuselage for shaping the overall cross area distribution, a technique which cannot be used by airliners with their cylindrical fuselages. Küchemann bodies, therefore, are a makeshift measure and only used when coke-botteling cannot be applied. Conformal tanks use the shortest possible fuel lines and add only very little friction drag, so they are a superior solution to Küchemann bodies when it comes to adding fuel volume.


text:
Why does setting the Legacy 600's transponder to standby mode also deactivate its TCAS?
In 2006, a 737 operating as Gol Transportes Aéreos Flight 1907 crashed following a collision with a Legacy 600 business jet; the Legacy's transponder proved to have been in standby mode1 at the time of the collision (due either to a transponder malfunction or to the crew having inadvertently switched it into standby mode), which, obviously, prevented the 737's TCAS from seeing the Legacy and sounding an alarm.
However, setting the Legacy's transponder to standby mode also, apparently, deactivated its own TCAS:

Considering, according to declarations, that the N600XL pilots did not perceive that the transponder had stopped transmitting and that, as a result, the TCAS was inoperative...  [CENIPA report, page 83.]

This is also, apparently, the result should the transponder fail outright:

In case there is a Transponder failure, the operation mode and code selected are replaced by dashes in the RMU. An orange message “TCAS FAIL” will appear on both Primary Flight Displays (PFD) precisely in front of the visual field of view of both pilots, as shown by the pictures bellow [sic].  [CENIPA report, page 92.]

I'm not seeing the rationale for this; even if the aircraft's transponder fails or is switched off, there doesn't seem to be any inherent reason why this should require also deactivating the TCAS.  Even if the transponder can't, or won't, transmit, the TCAS should still be able to pick up signals from other aircraft's transponders, allowing it to continue to function, albeit in a receive-only mode.
So why does the Legacy 600's TCAS cease to function if the transponder is standing by or inoperative?

1: In standby mode, the transponder does not respond to any interrogation signals, either from the ground or from other aircraft.

Anyone know why this has been voted down to -3? This seems like an utterly reasonable question to me.

@TannerSwett In my opinion the question could be greatly improved by making it far more concise and not making assumptions. Essentially the question asks why TCAS cannot operate without a Mode S transmitter. A single sentence could tie this into the accident. The opinion, a paragraph about "stupid design" could be omitted completely. As is, the question feels much more like a rant than a true technical query.

should still be able to pick up signals from other aircraft's transponders But TCAS determines the locations of other aircraft by sending its own interrigations after it hears the mode S address. It can't calculate range to a target just from hearing the other aircraft's mode S transmission. Also, RA's are calculated depending on cooperation between both aircraft. If one is silent then the system won't work.

At least under FAA regulations the TCAS system goes into standby (by design) if the transponder fails

A Mode S transponder is required to be installed and operational for
  TCAS II to be operational. If the Mode S transponder fails, the TCAS
  Performance Monitor will detect this failure and automatically place
  TCAS into Standby. The Mode S transponder performs the normal
  functions to support the ground-based ATC system and can work with
  either an ATCRBS or a Mode S ground sensor. The Mode S transponder is
  also used to provide air-to-air data exchange between TCAS-equipped
  aircraft so that coordinated, complementary RAs can be issued when
  required.

TCAS II systems get their altitude information from the transponder, which is a vital parameter to the system working.

(source)
All modes also require a functioning Mode S transponder

Stand-by: Power is applied to the TCAS Processor and the Mode S transponder, but TCAS does not issue any interrogations and the
  transponder will reply to only discrete interrogations. The
  transponder still transmits squitters. Note: If the aircraft is on the
  ground and transmitting extended squitters, it is not required to
  transmit short (acquisition) squitters.
Transponder: The Mode S transponder is fully operational and will reply to all appropriate ground and TCAS interrogations. TCAS
  remains in Stand-by.
TA Only: The Mode S transponder is fully operational. TCAS will operate normally and issue the appropriate interrogations and perform
  all tracking functions However, TCAS will only issue TAs; RAs will be
  inhibited.
Automatic or TA/RA: The Mode S transponder is fully operational. TCAS will operate normally and issue the appropriate
  interrogations and perform all tracking functions. TCAS will issue TAs
  and RAs when appropriate.

Strictly speaking all of this is only applicable to TCAS II systems but based on the noted case and my reading of the accident report it appears TCAS II was the system in use in this case.

Why does the TCAS get its altitude information from the transponder?  Why can't it get it directly from the altimeter?

@Sean if you dig into the document linked above it sounds like TCAS I did just that, they dont really offer an explanation as to why they changed for TCAS II but I would assume its to keep some kind of uniformity between ground and air reporting (might be a good topic for another question, Im happy to do more research)

Not seeing where the document says that; could you point out where it does?

@Sean I took a second quick look and cant find the exact section. I remember coming across it when researching this last night that some of the earlier systems were tied directly into the altimeters but for TCAS II the transponder became a requirement.

I don't understand the reason behind this choice, this document actually confirms "TCAS II and ATC systems use the altitude data provided by the aircraft’s transponder". So not only RA coordination cannot take place due to the transponder air-air datalink loss, but even uncoordinated RA and simple TA cannot be issued, because of the loss of the altitude source. I'd be curious to know why the TCAS is not fed directly by the ADC. This would have prevented such accidents, as well as the use of  an older TCAS compliant with ACAS I protocol.


text:
How do barometric altimetry systems compensate for nonstandard pressure lapse rates?
Barometric altimetry relies on atmospheric pressure dropping at a steady rate with altitude, starting from a measured ground-level pressure.  This, in turn, relies on atmospheric density dropping at a constant, steady rate with altitude, which it does not always do:

For instance, if the air close to the ground is much more humid (and, thus, less dense) than the air higher up, then the atmospheric density will drop at a slower-than-standard rate with altitude as the increasing rarefaction of the air is partially compensated by its decreasing humidity, causing the atmospheric pressure to fall off more slowly with altitude than it “should”, and causing an uncorrected altimeter to read falsely low.
Conversely, in a industrial area, with heavily-polluted low-level air containing high concentrations of dense gasses such as carbon dioxide, sulfur oxides, nitrogen dioxide, hydrogen chloride, sulfuric and nitric acids, and heavy hydrocarbons, plus lots of soot and tar particulates which raise the air’s mass significantly while adding very little additional volume, atmospheric density falls off faster with altitude than in a standard atmosphere, as one rises out of the pollution-densified lower layers; this causes the atmospheric pressure to lapse at a higher-than-standard rate, which would cause an uncorrected altimeter to read falsely high.

How do barometric altimeters compensate for nonstandard pressure lapse rates resulting from nonstandard atmospheric density profiles stemming from variations in atmospheric composition with altitude?

This is different from How does an altimeter deal with the non-linear pressure gradient?.  That question is about how an altimeter is designed to match the non-linear ISA model; my question is about how one deals with situations where the ambient air isn't the ISA.

@Ben I don't think it is a duplicate. That question is about the non-linearity, which is accounted for in the ISA and calibrated. This question asks about anomalies which aren't accounted for in the ISA.

This is not a duplicate of the linked question. The link asks about compensating for the non-linear nature of the mathematical pressure model. This question asked about non standard atmospheric conditions. And, I would like to answer it.

@DanHulme: I was about to explain why it isn't a duplicate, but you nailed it right off the bat.

i just flagged it for review to get it re-opened. I like some things about SE but the points based moderator system has issues with arbitrary actions.

They don't.
Since all aircraft in an area are (or at least should be) having roughly the same error, the errors cancel out and there is no practical effect on vertical separation.
Falsely low readings result in more terrain or obstacle clearance than needed, so that's not a problem either.
Falsely high readings do potentially create a problem, but the minimum clearance is deliberately large to account for the accepted range of altimetry error plus some additional buffer. For example, it doesn't matter if you're 100ft lower than you think when you're still 900ft (rather than 1000ft) above any hazards.

It's also crucial to mention that corrections are mandated during extremely cold weather operations (-30degC and lower, case of falsely high readings), by ATC or ATIS or other the Company or Manufacturer's instructions.
A chart/table "surface Temp vs altitude correction at different heights above airfield" is consulted to give the corrections and these are added to the Minimum charted altitudes in the Instrument Approach Procedures.
For a surface temperature of -40degC the correction at 3000 ft above aerodrome is 740 ft which all will agree is significant. AT -30degC it is 570 ft.
Such situations could be commonplace in Canada, Russia and the Scandi states.

And have been known to occur, though less than commonly, in places like Spokane, WA, Fargo, ND, Rochester or Buffalo, NY, etc.  Temperatures below -30C aren't all that rare in the northern parts of the Great Plains, even hundreds of kilometers from the Canadian border.  And that's not even to mention Alaska (which is short on major airports, but not on low temperature winter mornings).


text:
who can do an overhaul of landing gear?
A manufacturer requires the landing gear of an aircraft to be overhauled at a 10 year interval.
Can any maintenance facility perform this work or does it need to be an approved facility.
This is for a Part 91 only plane.

Hi Mark, welcome to aviation.stackexchange. I've added the FAA-regulation tag, as I assume you look for answers from the viewpoint of US legislation (you mentione Part 91). If you are looking for answer from the viewpoint of different legislations, please modify the tags accordingly

Exactly what document states 10 years? And what exactly does it say?  Recommended? Required? Mandatory? Generally in the US, the only aircraft-specific thing that's mandatory, especially for private a/c, is something specified in an AD, or on complex aircraft that have some kind of approved maintenance program.

When "the manufacturer requires {something}", they typically spell out the details like you're asking about. Go back to the source document that gives the requirement and see what it says. It may say "must be overhauled by the manufacturer" or "at any FAA-approved facility" or something else, and either way, you have your answer.

Overhauls are maintenance, and hence anyone can do them, as long as they have the right certificate (mechanic or repairman), or work under the supervision of one. The catch is who signs off on the major maintenance for return to service. For that you need someone with FAA inspection authorization, and no IA in his or her right mind will sign off on landing gear maintenance performed by a person or shop with no experience and no certificates or record of supervision.


text:
What are the advantages of a Spinning Wing (Magnus Effect) and why haven't any been commercially produced?
What were the advantages and why was it never produced?

complexity I guess

What makes you think there were any advantages?

It seems to me that the gyroscopic forces would be massive for such a big drum. Modern airplanes go through tires very often because they skid when initially touching the pavement. Pre-spinning the wheels to reduce skidding has proven to be infeasible because even the relatively small wheels have produced gyroscopic forces making maneuverability more difficult whilst landing.

This made me think, though, that perhaps a sort of cloth skin could go over the outside of the wing and be rotated so that there isn't a heavy cylinder, but instead a light piece of cloth that would wrap and spin around the wing. This would also provide more surface area to catch the air and the wings would still be functional in the case that the power source spinning this stops functioning. It could even be used just at takeoff for more lift, allowing for lift-off in shorter distances. This seems much more feasible in this way.

Applicable:  http://www.fanwing.com/
One of the claimed benefits is extremely low stall speed.  I have no idea, I just saw the site a few months ago and recognized your drawing above as what the site was promoting.

There are a lot of designs like the spinning wing and aerodyne that work just fine but which rely completely on having power for lift. That's a deal breaker because engines quit. Any design that MUST have power for lift will never be a primary system for any flying aircraft. The only reason we use helicopter is autorotation. We're it not for that, helicopters would be suicidal. Another example of 99% great killed by 1% deal breaking suck.

Some earlier prototypes of spinning-wing aircraft were produced but none were successful. The design has some very serious disadvantages. One of the main issues appears to be undesirable gyroscopic effects.

Production

... why it was never produced?

Several full-scale prototype aircraft with rotating Magnus-effect wings seem to have been produced:

921-V - Plymouth. 1930.
X772N - Union Aircraft Co. 1931.

It may be that 921-V is the only one to have flown, crashing after one flight.
 The Flettner airplane

Built in 1930 (USA), the 921-V is reported to have been flown at least once - ending it's short career with a crash landing. Three cylinders with disks performing as winglets driven by a separate engine. Information on this design needed! It's probably the only aircraft equipped with cylinder wings which made it into the air.

From Pilotfriend.com
In practice the effect can be less efficient than conventional alternatives

In the early 1920's the force from a rotating cylinder was used to power a sailing ship. The idea, proposed by Anton Flettner of Germany, was to replace the mast and cloth sails with a large cylinder rotated by an engine below deck. The idea worked, but the propulsion force generated was less than the motor would have generated if it had been connected to a standard marine propeller!

From NASA

Advantages

What were the advantages?

The article referenced suggests

More wing lift and less drag are the major aims of aviation’s researchers. Maybe the Magnus Wing will supply the answers ... The ship is powered by a conventional gas-turbine jet engine, while the drums are revolved by a separate piston engine. Small wings, heavy loads and quick take-offs would be its great advantages.

Disadvantages
The developer of a working model wrote about a few of the disadvantages (try a google translation of that page for more):

If the cylinder rotation accidentally becomes slow or stops, its lift disappears completely. This plane will never be able to glide.
If a gust blows from backside during a slow flight (takeoff or landing), the cylinder wings generate a down force.
The spinning cylinder wings generate a strong gyro effect, which makes it difficult for the plane to change its attitude.

See video and comments.

NASA did some experiments with rotating-cylinders for flaps (not main lift source)

They concluded

These experiments demonstrated on the one hand the
  effectiveness of such a high lift system but on the other hand the weakness in handling qualities due to the gyroscopic forces in such an aircraft configuration.

From A review of the Magnus effect in aeronautics

The US Army also carried out a study of the use of the Magnus effect in aircraft
VIEW AND PRELIMINARY EVALUATION OF LIFTING HORIZONTAL-AXIS ROTATING-WING AERONAUTICAL SYSTEMS

It is difficult to pick out a simple conclusion as the study looked at a wide variety of systems. For rotating cylinders in wing (RCIW) they wrote

Such systems do not appear to have merit as STOL devices.

Thanks @RedGrittyBrick again, you're the one which answers almost all of my question professional and good

Great answer! You have made this page the definite source for Magnus effect planes on the whole Internet.

I would add that Magnus-effect wings don't seem to provide better efficiency. Induced drag would still be the same (and it is inversely proportional to wing span, so making the wing shorter only makes things worse) and being thicker it does not look like they should have lower form drag.

Couldn't you counter the gyroscopic forces with another cylinder inside the first one spinning in the opposite direction? The article seems to claim that turbulence would be reduced or even eliminated. Wouldn't the extra difference between the top and bottom airflow speeds increase turbulence?

@CJDennis: I think you could, see Dynamics of counter-rotating flywheels. That adds a lot to complexity and weight. You'd need much stronger framework and bearings to keep the two cylinders separate as the aircraft banked, yawed or pitched up or down. Any failure might be spectacular.

This has blown my mind! I love the idea of a magnus effect jet too! I might be getting the angular momentum vector wrong, but won't the spinning cylinders dampen the effect of turbulence on the airframe?

@shortstheory: It probably would make things worse, gyroscopic effect. To dampen a force, the reaction force would have to oppose the action force. With gyroscopes, the reaction force is orthogonal to the action force and the axis of rotation. With this arrangement, the axis of rotation is pitch so the gyroscope couples roll and yaw.

Note that the prototype YOV-10A pictured above was housed at the Yankee Air Museum at Willow Run Airport in Michigan, US until it was destroyed by fire October 9, 2004. http://www.ov-10bronco.net/news_detail.cfm?NewsID=319

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
Could bypass air be used to cool a rocket engine like in turbofans?
Rocket engines typically employ a cooling jacket of sorts, usually dual-purposing the fuel as a liquid coolant before it flows into the injectors.
Here is an example turbojet using a bypass. As you can see, the bypass air is far cooler in temperature than the air combustion chamber:
Source
Why are there no designs that use an air bypass (similar to a turbojet) to cool the engine/nozzle? Obviously once the rocket has escaped the atmosphere, a bypass would be of little use. However usually by that stage in a rocket, thrust has been greatly reduced and therefore generated heat as well.
Wouldn't a bypass cooling system make a rocket engine a tad less complex (especially for the first stage)?

On a rocket, where is this bypass air going to come from? Also, fuel has a much higher heat capacity than air.

@fooot I imagine vents near/around the nose cone to open the bypass. Since the rocket is traveling very fast, it would ram air through the bypass without a fan being necessary.

How would this be less complex than the current approach?

@CareyGregory Maybe I don't understand something - but it seems heating fuel rapidly is inherently dangerous, and it requires extra high pressure plumbing, etc. If the top of the rocket just had openings that allowed air to travel inside the shell and down around the nozzle (carrying heat away with it), then we're just talking about some ducting.

@SnakeDoc Heating fuel isn't as dangerous as you might think, as long as you don't combine it with an oxidizer. Even aircraft that fly in air frequently make use of the high heat capacity of their fuel. It's orders of magnitude higher than what you'll get from air, especially at high altitudes, and its effectiveness will be more or less constant, rather than decreasing rapidly with altitude. Furthermore, rocket engines used in a space launch will generally be releasing orders of magnitude more energy than a turbofan jet engine would be.

@SnakeDoc: Rocket fuel cannot burn without oxygen (at least, not at the temperatures we're talking about. Obviously anything would burn with or without oxygen when thrown into the sun). If the plumbing leaks and oxygen enters the fuel lines you have bigger problems

This is not really air cooling, but some engines use curtain cooling where relatively cool exhaust gases from the turbopumps are fed to part of the nozzle. See https://en.wikipedia.org/wiki/Rocket_engine#Cooling .

@reirab That is interesting indeed. I would have thought taking rocket fuel (say Liquid Hydrogen) from -253 degrees C and touching it to something that's heated up to 1200+ degrees C, would be enough to "flash ignite" the fuel.

@SnakeDoc In the presence of oxidizer, perhaps. Nothing combusts without oxidizer, though, regardless of temperature. (The 'burning' in the sun slebetman mentioned is not combustion; it's nuclear fusion.) Combustion is a chemical reaction (specifically, a redox reaction.) A chemical reaction can't take place without all of the reactants being present.

@reirab That's pretty good info. You should put it down in an answer so I can upvote it.

Virtually all jet engines use a technique very similar to the technique you're proposing: the inside of the combustion chamber, turbines, and exhaust, (and afterburner, if the engine has one) as well as the turbine blades and stators, are covered with small holes. Cooling air is forced through these holes, forming a boundary layer between the metal and the super-hot gases.
The temperature of burning fuel/air in a jet engine is well above the melting point of the metals used to build these components, which is why this is necessary. In fact, the temperature of the gases exceeds the vaporization temperature of a number of metals.
Where does this cooling air come from? It comes from the engine's compressor. The reason bypass air isn't used is because it is at a lower pressure than the inside of the core of the engine, so the hot gases would move out through the holes toward the bypass duct. In this case, the boundary layer is made of hot gas instead of cool gas, which destroys the engine.
To get air of adequate pressure, air is "bled" out of the compressor at points where the pressure is high enough to provide adequate flow (in the right direction!) to the parts of the engine that need it. This method of cooling is so effective that the components in a jet engine that need to withstand the most heat are actually the highest pressure parts of the compressor, because (a) air heats up when you compress it, and (b) you can't use the same cool boundary layer technique in the compressor because you need higher pressure air than the hot air from which you're trying to protect the component, and the component is part of the highest-pressure part of the compressor.
Here's a picture of a jet exhaust guide vane, with cooling holes clearly visible:

On a rocket engine, you could potentially cool it with compressed atmospheric air, but doing would add weight and complexity, and would only work in the lower layers of the atmosphere where there's enough air density to provide an adequate mass flow rate to absorb the heat. Since, as you mentioned, fuel (or potentially oxidizer) can be used to effectively cool the engine for a much lower penalty in weight and greater freedom in operating environment, that's been the obvious choice for most high-performance liquid fueled rockets.

The air augmented rocket uses air from the freestream to provide extra thrust (and potentially) cool the motor. Two of the main issues with trying to add 'extra' air into the rocket stream are: drag over the rocket body (pretty much the main one) and disruption of the high-temperature flow (especially if you are trying to introduce air somewhere around the nozzle).

Ah, somebody has tried this before - NASA even. I didn't think disrupting the high-temperature flow would be an issue, since the thrust is being generated inside the nozzle.

Disrupting the jet flow is mostly a concern if you are trying to incorporate the freestream air into the nozzle

Two main reasons: Firstly rockets operate at a preposterous level of power - the fuel and oxidiser pumps alone on a space shuttle have a combined shaft power greater than a GE90 turbofan - so you'd really struggle to get enough airflow, liquid hydrogen is a far better heatsink especially in the quantities they use.
Secondly, most rockets are built with the intention of going to space, where there in no cooling air at all, so as you got higher and the air got thinner your air-cooled rocket would rapidly overheat.

There is a potential bonus though - entrained air flow could be used to increase thrust in the similar way to how a bypass fan works - The total thrust is $ \sqrt(2 \nu P ) $ where $\nu$ is the total exhaust effective mass flow straight backwards, and $P$ is the power input, from fuel. If the mass rate can be increased as with a bypass fan, the thrust can increased. This could significantly reduce fuel required in early stage, where fuel use is greatest. The effective exhaust speed is reduced, but the increase in mass flow more than compensates.